Treatment or isolation of vascular aneurysms or of vessel walls which have been thickened by disease has traditionally been performed via surgical bypassing with vascular grafts. Shortcomings of this invasive procedure include the morbidity and mortality associated with major surgery, long patient recovery times, and the high incidence of repeat intervention needed due to limitations of the graft or of the procedure.
Minimally invasive alternatives involving stents or stent-grafts are generally known and widely used in certain types of treatments. Intralumenal stents, for example, are particularly useful for treatment of vascular or arterial occlusion or stenosis typically associated with vessels thickened by disease. Intralumenal stents function to mechanically hold these vessels open. In some instances, stents may be used subsequent to or as an adjunct to a balloon angioplasty procedure.
Stent-grafts, which include a graft layer either inside or outside of a stent structure, are particularly useful for the treatment of aneurysms. An aneurysm may be characterized as a sac formed by the dilatation of the wall of an artery, vein, or vessel. Typically the aneurysm is filled with fluid or clotted blood. The stent-graft provides a graft layer to reestablish a flow lumen through the aneurysm as well as a stent structure to support the graft and to resist occlusion or restenosis.
Treatment of a bifurcation site afflicted with such defects as an occlusion, stenosis, or aneurysm is a particularly demanding application for either stents or stent-grafts. A bifurcation site is generally where a single lumen or artery (often called the trunk) splits into two lumen or arteries (often called branches), such as in a "Y" configuration. For example, one such bifurcation site is found within the human body at the location where the abdominal aortic artery branches into the left and right (or ipsalateral and contralateral) iliac arteries.
When a defect, such as an aneurysm, is located very close to the bifurcation of a trunk lumen into two branch lumens, treatment becomes especially difficult. One reason for this difficulty is because neither the trunk lumen nor either of the branch lumens provides a sufficient portion of healthy, lumen wall on both sides of the defect to which a straight section of single lumen stent or stent-graft can be secured. The stent or stent-graft must span the bifurcation site and yet allow undisturbed flow through each of the branch and trunk lumens.
What is required then is a stent or stent-graft which may be secured to each of the lumen wall a sufficient distance away from the defect and yet is capable of allowing undisturbed flow into each of the branch and trunk lumen. Such a configuration, at least after implantation, generally must have the same Y-shape as described for the bifurcation site. Prior to implantation, the stent or stent-graft may have a Y-shape or may have a modular construction which is assembled into the desired shape as it is implanted.
As we shall see, deployment of implants adapted to meet these needs is also problematic in that they must be deployed and secured in three different lumen which are impossible to access from a single direction. Further, to facilitate intralumenal delivery through a body's tortuous vasculature, the implant must be capable of being compressed into a very small diameter or profile and then expand to a predetermined geometry adapted to engage the vessel wall. However, sometimes this expanded condition is insufficient to completely occlude flow over the device and into the endolumenal defect.
Prior devices that deal with treatment at a bifurcation site within the body generally include grafts, stents, and stent-grafts in either a single-piece or modular configuration.
The use of tubular grafts for treating defects at bifurcation sites has been known for some time. For example, in U.S. Pat. No. 3,029,819 to Starks, U.S. Pat. No. 3,096,560 to Liebig, U.S. Pat. No. 3,142,067 to Liebig, and U.S. Pat. No. 3,805,301 to Liebig. These grafts are typically made of woven fabric or other synthetic material and, because they have no supporting stent structure, typically involve excising the defected segment and suturing the fabric graft in place using common surgical procedures.
A number of bifurcated graft implants have been developed which use some limited means of supporting the one-piece bifurcated graft structure. Examples of such bifurcated grafts include U.S. Pat. No. 4,562,596 to Kornberg; U.S. Pat. No. 5,489,295 to Piplani et al.; and U.S. Pat. Nos. 5,360,443, and 5,522,880, both to Barone et al.
As with all such one-piece devices, the delivery of the graft implant is complicated by the fact that each of the trunk and two legs of the graft must be positioned into their respective lumen and then secured into place. This requires the branch legs to be compressed together for delivery through one of the lumen and requires difficult maneuvering of the branch legs to get them unfolded and untwisted into place within their respective branch lumen. This type of delivery requires the graft sections to be highly flexible so that its components may be manipulated as required and requires a larger profile. This demand for high flexibility often results in unsupported graft sections that may be subject to kinking, folding, collapse or the like.
Bifurcated stent devices have also been disclosed, such as in U.S. Pat. No. 4,994,071 to MacGregor, for example, which discloses a single-piece bifurcated stent for insertion into a bifurcating vessel, or in U.S. Pat. No. 5,342,387 to Summers.
Some implant devices have further used a modular approach, primarily for purposes of enhancing delivery. Examples of modular implants such as stents or grafts include FR 2 678 508 A1; U.S. Pat. No. 5,507,769 to Marin et al; EP 0 (551) (179) A1 to Palmaz et al.; WO 95/(215)92 (International Application number PCT/US95/(014)66); EP (068)6 (379) A2; EP 0 (696) (447) A2; and U.S. Pat. No. 5,562,724 to Vorwerk et al. While these modular devices tend to offer a measure of improved delivery, continuing problems may include a certain amount of leakage around the openings of the device or at the modular connection, as well as increased compressed profiles, and inoptimal flexibility, kink-resistance, and axial stiffness.
Another problem associated with the endovascular repair of aneurysms is postprocedural leakage into the aneurysm sac. Katzen et al., Initial Experience Performing Combined Surgical/Interventional Procedures in the Interventional Suite (1996) J. Endovasc. Surg. 3: 467-468, discloses the treatment of patients for abdominal aortic aneurysms (AAAs) using covered (Dacron or polytetrafluoroethylene) multisegment Z-stents; approximately one third of the patients experienced postprocedural leakage. Repair cases of AAAs using the White-Yu endovascular graft were described in White et al., Endoleak Following Endoluminal Repair of AAA: Diagnosis, Significance, and Management (1996) J. Endovasc. Surg. 3: 339-340; this technique resulted in leakage around the graft 7.8% of the time. Chuter et al., Bifurcated Stent-Grafts for AAA: 3-Year Follow-Up (1996) J. Endovasc. Surg. 3:453, describes the observation of persistent perigraft leakage 12% of the time in a late portion of patients treated for an AAA using a bifucated stent-graft. Parodi et al. Long-term Follow-up of AAA Endoluminal Repair (1996) J. Endovasc. Surg. 3:335, cites leakage as one of the primary factors causing early failures of aortic tube graft replacement treatment of AAAs, employing either proximal stent fixation or aortoiliac stent grafts; perigraft leakage is the only cited cause in late failures.
The postprocedural leakage problem has persisted in more recently developed systems for AAA treatment. Moore et al., in Transfemoral Endovascular Repair of Abdominal Aortic Aneurysm: Results of the North American EVT Phase 1 Trial (April 1996) J. Vasc. Surg., 543-552, disclose an endovascular grafting system (EGS) consisting of an endovascular prosthesis, an endovascular deployment assembly, and an expandable introducer sheath developed by Endovascular Technologies for the treatment of AAAs. Moore et al. discloses postprocedural leakage for this device, as initially detected, in 44% of the patients; persistent leakage was observed in greater than 20% of the patients.
An endovascular grafting system is further described in Dereume et al. Endoluminal Treatment of Abdominal Aortic Aneurysm with the Corvita Endovascular Graft. Results of a Single-Center, Prospective Feasibility Study of 90 Patients (1996) J. Endovasc. Surg. 3:453-483. Dereume et al. describe a system composed of a metallic self-expanding braided wire stent and an inner liner comprised of polycarbonate urethane microfibers. Among patients treated with this graft, 38% presented some postprocedural leakage, according to the Dereume et al. disclosure.
In short, the prior art does not disclose a system for the endovascular repair of AAAs that has adequately addressed the problem of postprocedural leakage.
From the foregoing discussion it is evident that it would be desirable to have a stent-graft device for treating vessel wall aneurysms by endolumenally isolating the abnormal aneurysmal wall from blood pressure, and which does not allow for substantial leakage flow around the outer periphery of the device.